The use of a RuAl seed layer, which is included in the preferred embodiment discussed below, is described in a commonly assigned, co-pending U.S. patent application with Ser. No. 09/295,267. The use of an onset layer, which is included in the preferred embodiment discussed below, is described in a commonly assigned, co-pending U.S. patent application with Ser. No. 08/976,565 entitled xe2x80x9cThin Film Disk with Onset Layer.xe2x80x9d U.S.P.T.O application Ser. No. 09/020,151, entitled xe2x80x9cTHIN FILM MAGNETIC DISK HAVING REACTIVE ELEMENT DOPED REFRACTORY METAL SEED LAYERxe2x80x9d is mentioned below.
1. Field of the Invention
This invention relates generally to the field of thin film materials used in magnetic disks for data storage devices such as disk drives. More particularly the invention relates to the use of an improved onset layer in between an underlayer and magnetic layer on a thin film disk.
2. Background of the Invention
The magnetic recording disk in a conventional drive assembly typically consists of a substrate, an underlayer consisting of a thin film of chromium (Cr) or a Cr alloy, a cobalt-based magnetic alloy deposited on the underlayer, and a protective overcoat deposited on the magnetic layer. A variety of disk substrates such as NiP-coated AlMg, glass, glass ceramic, glassy carbon etc., are used. Disks that are commonly available in the market are made with an AlMg substrate on which a layer of amorphous NiP is electrolessly deposited. While a coating on the substrate is important because such a coating gives uniform magnetic read-back signals during the course of a disk revolution, the process of electroless deposition of NiP on an AlMg substrate has several disadvantages, one of them being the fact that electroless deposition is a wet process. The wet nature of the process necessitates that it be performed quite separately from the sputtering process by which the remainder of the layers in a magnetic recording disk is deposited. A NiP layer has other disadvantages too. For instance, with a NiP layer, it is difficult to achieve the smoothness and uniformity in the NiP surface of the magnetic recording disk, which is a prerequisite for the high densities required in current disk drives. Yet another problem associated with the NiP surface is corrosion. The NiP surface also tends to limit the processing temperatures because of its tendency to become magnetic if heated beyond a certain point.
Further, in cases where a non-metallic substrate such as glass is chosen, the conventional NiP coating is not preferable for use on glass as pre-seed layer for many reasons including those noted above. In such cases, the non-metallic substrate disks typically have a so called xe2x80x9cseed layerxe2x80x9d sputter deposited onto the substrate between the substrate and the Cr-alloy underlayer. The selection of the seed layer allows the performance of non-metallic substrates to exceed the magnetic recording characteristics of NiP/AlMg disks because the seed layer of the magnetic disk drive influences nucleation and growth of the underlayer which in turn affects the recording characteristics of the magnetic layer. Several materials have been proposed in published papers for seed layers such as: Al, Cr, CrNi, Ti, Ni3P, MgO, Ta, C, W, Zr, AlN and NiAl on glass and non-metallic substrates. (See for example, xe2x80x9cSeed Layer induced (002) crystallographic texture in NiAl underlayers,xe2x80x9d Lee, et al., J. AppI. Phys. 79(8), Apr. 15, 1996, p.4902ff). In a single magnetic layer disk, Laughlin, et al., have described use of a NiAl seed layer followed by a 2.5 nm thick Cr underlayer and a CoCrPt magnetic layer. The NiAl seed layer with the Cr underlayer was said to induce the [1{overscore (01)}0] texture in the magnetic layer. (xe2x80x9cThe Control and Characterization of the Crystallographic Texture of Longitudinal Thin Film Recording Media,xe2x80x9d IEEE Trans. Magnetic. 32(5) September 1996, 3632). In one of the related applications noted above, the use of RuAl for a seed layer is disclosed.
A Cr underlayer is mainly used to influence such microstructural parameters as the preferred orientation (PO) and grain size of the cobalt-based magnetic alloy forming the onset layer. When the Cr underlayer is deposited at elevated temperature on a NiP-coated AlMg substrate a [100] PO is usually formed. A PO of the underlayer promotes the epitaxial growth of [1{overscore (12)}0] PO of the hcp cobalt (Co) alloy forming the onset layer, thereby improving the in-plane magnetic performance of the disk for longitudinal recording. The [1{overscore (12)}0] PO refers to a film of hexagonal structure whose (1{overscore (12)}0) planes are predominantly parallel to the surface of the film. Since nucleation and growth of Cr or Cr alloy underlayers on glass and most non-metallic substrates differ significantly from those on NiP-coated AlMg substrates, different materials and layer structures are used on glass substrate disks to achieve optimum results.
The use of an onset layer has already been described in U.S. Pat. No. 5,736,262 in which a wide range of CoCr compositions were claimed including both ferromagnetic and non-magnetic alloys. The patent mentions that a non-magnetic onset layer is preferred because such a non-magnetic onset layer insures that the onset layer does not in any way contribute towards the magnetic properties of the disk. U.S. Pat. No. 5,922,442 specifies an onset layer with a defined saturation magnetization. However, the patent is silent about the desired Cr concentration of the onset layer. Since a magnetic layer of CoPtCrB alloy is difficult to grow epitaxially on a Cr alloy underlayer, there exists a need for an onset layer, which is conducive for the growth of magnetic layers.
The design of magnetic disks has advanced rapidly in recent years and, even 1 dB improvement in the Signal-to-Noise Ratio (SNR) is now considered quite significant. Recording density of magnetic disks as high as 30 to 40 gigabits per square inch has been achieved in the industry. However, this density has only been achieved in the laboratory and the density found in state of the art commercially available disk drives is far below this value. The density of a disk is also dependent on the thermal stability of the recorded information on the disk because a commercially viable disk drive must be capable of maintaining the stored information for periods of time measured in years.
The thin film disk of the invention includes a thin film pre-seed layer of amorphous or nanocrystalline structure. The pre-seed layer, which may be chromium-tantalum CrTa or aluminum-titanium (AlTi) or aluminum tantalum (AlTa) is deposited prior to a first crystalline layer. Although the pre-seed layer may be amorphous or nanocrystalline, for brevity it will be referred to herein as amorphous which is intended to encompass a nanocrystalline structure. In the preferred embodiment of the present invention, a pre-seed layer is sputtered onto a non-metallic substrate such as glass, followed by a ruthenium-aluminum (RuAl) seed layer with B2 structure. The use of the pre-seed layer improves grain size and its distribution, in-plane crystallographic orientation, coercivity (Hc) and SNR. In a preferred embodiment of the present invention, the pre-seed layer is followed by a RuAl seed layer, a Cr alloy underlayer, an onset layer and a magnetic layer. The amorphous pre-seed layer also allows use of a thinner RuAl seed layer which results in smaller overall grain size, as well as, a reduction in manufacturing cost due to relatively high cost of ruthenium. The increased coercivity also allows the use of a thinner Cr alloy underlayer, which also results in smaller overall grain size. Another benefit lies in the fact that the pre-seed layer provides additional thermal conductivity, which could help prevent thermal erasures on a glass disk. In accordance with a preferred embodiment of the present invention, an onset layer is used with an optimal concentration of Cr and an optimal thickness adapted to increase coercivity and improve the Signal-to-Noise Ratio (SNR).